Property change listener efficiency changes

-   Using direct non-enumerable `__` prefixed properties instead of weak
    maps for fast access to change listener records and overridden
    property descriptors.
-   Method names and reusable active dispatch array now kept in book
    keeping to avoid recomputation.
-   Presumed hot dispatch loop factored into separate method to isolate
    from try/catch deoptimization.
-   Removed support for uninstalling property change listeners.
Could you explain a bit more what `current` and `active` are used for?